Yes, and the bottom line is that the weight savings are negligible.

Think about it this way - at 80 degrees, air weighs about 1 kg/cubic meter. The same amount of water weighs 1000 kg (fresh water).

So, say a Hobie 16, fully submerged, displaces 1 cubic meter (1000 kg or 2200 lbs of water). If you were somehow able to maintain a vaccuum inside the hulls, they would only weigh 1 kg less.

Filling the hulls with Hydrogen / Helium would weigh more and be very hard to keep inside (the small molecules have a tendency to sneak out of the tiniest holes - the reason why rubber Helium baloons go flat after a while - the helium sneaks out between the large rubber molecules).